The present invention relates to a retractable sliding door and especially to a retractable door having a constant force spring which allows the door to be opened with a constant force over its sliding range.
In the past, there have been a number of closure mechanisms for sliding doors which have included various types of spring mechanisms including a spring biased reel having a band wound on the outer surface of the reel which is connected to the door. In these prior closure mechanisms, the spring is so wound that when the sliding door is opened, the flexible band plays off the reel and the tension of the spring increases. Upon release of the door, the force of the spring winds in the flexible band and pulls the door to a closed position. Because of the distance in which the springs have been extended and the increased spring tension as the door is pulled to a fully opened position, the doors require a much greater force to open wide and, when released, are returned by the spring with a much greater force which tends to slam the door shut. This has generally made a retractable door closure impractical. Some of the prior art devices have had mechanisms for adjusting the tension of the springs. Such mechanisms tend to have various disadvantages including being costly to manufacture and some have springs that slip so that the door does not fully close each time.
Prior art U.S. patents which provide for closure mechanisms for sliding doors can be seen in U.S. Pat. No. 3,246,363 to Rogas et al. which has a coil spring return door closure mechanism provided with means for adjusting the tension of the spring including a ratchet wheel affixed to the shaft which carries the coil spring in the closure and a pawl on the front wall of a housing of the closure which engages the ratchet wheel to provide a positive locking mechanism for the spring adjusting means. In the Demukai patent, No. 4,301,623, a semi-automatic sliding door device has a tension spring and is equipped with a braking device to prevent the rapid return of the door. The braking device includes permanent magnets on the outer framework and a braking plate made of copper or aluminum sheets on the sliding door so that the length of elongation of the tension spring can be reduced relative to the length of the sliding door. In the Wartian U.S. Pat. No. 3,267,513, a closure for a sliding door is provided which controls the closing movement of the door to avoid slamming or incomplete closing of the door. The control is achieved with a coil spring and pneumatic bleed closing system in which the coil spring and pneumatic bleed are independently adjustable without disassembly of the door closer. The Pittenger U.S. Pat. No. 2,992,450, is also a door closure mechanism for use with sliding doors which uses a rotatable mounted reel which carries a cable which extends from the housing for attachment to the frame of the door and which automatically varies the angle of the component of force exerted by the door closing mechanism as the door progresses along its path of travel so that the door is caused to close gently without slamming. In this patent, the coil spring and the reel that it rides on is attached to the door and moves with the door with the end of the spring loaded coil attached to the frame so that the angle of the cord extending from the reel attached to the door and the frame changes as the door is opened or closed.
In the Bundschuh patent, No. 4,675,938, a retractable device for doors or windows is provided for automatically closing the door after the door has been opened. This retractable device includes a drum having a coil spring power assembly for driving the drum along with a centrifugal brake for controlling the power assembly in the drum to eliminate slamming of the door during closing. A gear train transmission assembly connects the power assembly in the centrifugal brake. In the Matthews, U.S. Pat. No. 3,480,227, a retractable device with a compensating brake is used as a retractor for sliding screen doors or the like. The amount of braking force applied is controlled by the amount of tension on the cable to prevent the tendency of a spring to slam a door shut. The greatest braking action is applied at the start of the retraction of the door when the force of the spring is greatest and reduced as the spring action force diminishes.
The present invention uses a simplified constant force spring incorporated into a retractable sliding door mechanism which prevents the slamming of the door and does not increase the force in opening the door as the door is opened wider and which can have the constant force spring capacity multiplied by using two or more springs in tandem. The spring uses a band of steel tightly wrapped on a rod and heat treated to develop an inherent stress or memory to control the force of the spring during reeling and unreeling.